U.S. patent number 10,006,169 [Application Number 15/034,453] was granted by the patent office on 2018-06-26 for method for delignifying and bleaching pulp.
This patent grant is currently assigned to Evonik Degussa GmbH. The grantee listed for this patent is Evonik Degussa GmbH. Invention is credited to Thomas Dietz, Ralf Grimmer, Bernd Hopf, Verena Illhardt, Sigrid Wegmann.
United States Patent |
10,006,169 |
Dietz , et al. |
June 26, 2018 |
Method for delignifying and bleaching pulp
Abstract
Process for the delignification and bleaching of pulp,
comprising a first bleaching stage with hydrogen peroxide in the
presence of a molybdate or tungstate in an acidic aqueous mixture;
subsequent to the first bleaching stage, a second bleaching stage
with hydrogen peroxide in an alkaline aqueous mixture; and,
subsequent to the second bleaching stage, a third bleaching stage
with hydrogen peroxide in the presence of a molybdate or tungstate
in an acidic aqueous mixture.
Inventors: |
Dietz; Thomas (Haibach,
DE), Hopf; Bernd (Kahl, DE), Grimmer;
Ralf (Freigericht, DE), Wegmann; Sigrid
(Freigericht, DE), Illhardt; Verena (Schimborn,
DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Evonik Degussa GmbH |
Essen |
N/A |
DE |
|
|
Assignee: |
Evonik Degussa GmbH (Essen,
DE)
|
Family
ID: |
51743424 |
Appl.
No.: |
15/034,453 |
Filed: |
October 15, 2014 |
PCT
Filed: |
October 15, 2014 |
PCT No.: |
PCT/EP2014/072096 |
371(c)(1),(2),(4) Date: |
May 04, 2016 |
PCT
Pub. No.: |
WO2015/067446 |
PCT
Pub. Date: |
May 14, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20160298294 A1 |
Oct 13, 2016 |
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Foreign Application Priority Data
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Nov 6, 2013 [DE] |
|
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10 2013 222 468 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21C
3/026 (20130101); D21C 9/147 (20130101); D21C
3/00 (20130101); D21C 9/1057 (20130101); D21C
9/16 (20130101); D21C 9/166 (20130101); D21C
9/1042 (20130101); D21C 9/163 (20130101); D21C
9/1036 (20130101); D21C 9/10 (20130101) |
Current International
Class: |
D21C
9/16 (20060101); D21C 9/10 (20060101); D21C
9/147 (20060101); D21C 9/02 (20060101); D21C
3/00 (20060101); D21C 3/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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CA |
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101792851 |
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CN |
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279 703 |
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CZ |
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44 46 122 |
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Jun 1996 |
|
DE |
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10 2008 040 884 |
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Feb 2010 |
|
DE |
|
0 765 842 |
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|
EP |
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2 320 946 |
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FR |
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2969668 |
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FR |
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2 065 628 |
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JP |
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1836465 |
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2 176 677 |
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RU |
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WO 95/35406 |
|
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WO |
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WO 96/01921 |
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WO |
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WO 96/37654 |
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WO |
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WO 2009/108111 |
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Sep 2009 |
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WO |
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WO 2009/133053 |
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Nov 2009 |
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WO |
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Other References
Bandeira et al., in "ECF Bleaching with Molybdenum Activated Acid
Peroxide and Its Impact on Eucalyptus Pulp Properties and Effluent
Quality," Natural Resources 2, pp. 61-70 (Year: 2011). cited by
examiner .
International Search Report for PCT/EP2014/072096 filed Oct. 15,
2014. cited by applicant .
English language translation of the Written Opinion of the
International Searching Authority PCT/EP EP2014/072096 filed Oct.
15, 2014. cited by applicant .
International Preliminary Report on Patentability for PCT/
EP2014/072096 filed Oct. 15, 2014. cited by applicant .
Search Report for corresponding German application 10 2013 222 468
dated Jun. 11, 2013 with English language machine translation
attached. cited by applicant .
Atia, "Adsorption of chromate and molybdate by cetylpyridinium
bentonite," Applied Clay Science 41:73-84 (2008) (available online
Oct. 2007). cited by applicant .
Database WPI Week 200165; Thomson Scientific, London, GB; AN
2001-577936; XP002540018 (Aug. 2001). cited by applicant .
Green, et al., Perry's Chemical Engineers' Handbook, Eighth
Edition; chapters 18.9 and 20.3.3; McGraw-Hill Education, published
as an ebook Nov. 2007. cited by applicant .
Kubelka, et al.,"Delignification with Acidic Hydrogen Peroxide
Activated by Molybdate," Journal of Pulp Paper and Science
18(3):J108-J114 (May 1992). cited by applicant .
Manning, et al., "Addition of Hydrogen Peroxide and Molybdate to
Chlorine Dioxide Bleaching Stages," Journal of Pulp Paper and
Science32(2):58-62 (Apr./May/Jun. 2006). cited by applicant .
Milagres, et al., "Bleaching eucalyptus pulps with short
sequences," Proceedings 3rd ICEP--International Colloqiuin on
Eucalyptus Pulp, pp. 1-5 (Mar. 2007) with English language
translation attached. cited by applicant .
Sameer, et al., "Recovery of Catalytic Molybdate from Dilute
Solutions," In the proceedings of the 93.sup.rd annual conference
of the PATPTAC, pp. A261-A268 (Apr. 2007). cited by applicant .
Sameer, et al., "Recovery of Molybdate from Dilute Aqueous
Solutions by Complexation with Cationic Surfactants and Extraction
with Isobutanol," Ind. Eng. Chem. Res. 47:428-433 (2008),
(published online Dec. 2007). cited by applicant .
Packham, et al., "Water Clarification by Flotation-1," The Water
Research Association, Technical Paper TP 8.7, Nov. 1972. cited by
applicant.
|
Primary Examiner: Fortuna; Jose A
Attorney, Agent or Firm: Law Office of Michael A. Sanzo,
LLC
Claims
The invention claimed is:
1. A process for the delignification and bleaching of pulp,
comprising: a) a first bleaching stage in which pulp is reacted
with hydrogen peroxide in the presence of a molybdate or tungstate
in an acidic aqueous mixture; b) subsequent to the first bleaching
stage, a second bleaching stage in which pulp is reacted with
hydrogen peroxide in an alkaline aqueous mixture; and c) subsequent
to the second bleaching stage, a third bleaching stage in which
pulp is reacted with hydrogen peroxide in the presence of a
molybdate or tungstate in an acidic aqueous mixture; wherein said
process does not comprise a bleaching stage in which an oxidizing
agent other than hydrogen peroxide and oxygen is used.
2. The process of claim 1, wherein in the first and third bleaching
stages, the pulp is reacted with 0.1 to 5 wt % of hydrogen peroxide
in the presence of 10 to 2000 ppm of molybdenum in the form of
molybdate or 200 to 10 000 ppm of tungsten in the form of
tungstate, based in each case on the mass of dry pulp, at a
temperature of 50 to 150.degree. C. and at a pH in the range from 1
to 7, and in the second bleaching stage the pulp is reacted with
0.1 to 5 wt % of hydrogen peroxide, based on the mass of dry pulp,
at a temperature of 50 to 100.degree. C. and at a pH in the range
between 7 and 12.
3. The process of claim 1, wherein the pulp is washed after the
first bleaching stage and after the second bleaching stage.
4. The process of claim 1, wherein said process does not comprise a
further bleaching stage in which an oxidizing agent other than
hydrogen peroxide and oxygen is used.
5. The process of claim 1, wherein the second bleaching stage is
carried out with addition of oxygen at a pressure of 0.1 to 1.5
MPa.
6. The process of claim 1, wherein a complexing agent is added in
the first bleaching stage, in the third bleaching stage or in the
first and third bleaching stages.
7. The process of claim 1, comprising an additional stage of
alkaline delignification of the pulp with oxygen before the first
bleaching stage.
8. The process of claim 1, comprising an additional bleaching stage
in which pulp is reacted with hydrogen peroxide in an alkaline
aqueous mixture after the third bleaching stage.
9. The process of claim 1, comprising an additional stage of acidic
hydrolysis with addition of a complexing agent before the first
bleaching stage.
10. The process of claim 1, comprising the additional steps of: a)
separating the pulp from the aqueous mixture after the first
bleaching stage, the third bleaching stage or the first and third
bleaching stages to give a molybdate- or tungstate-containing
aqueous solution; b) contacting the molybdate- or
tungstate-containing aqueous solution obtained in step a) with a
water-insoluble, cationized inorganic carrier material at a pH in
the range between 2 and 7, to give a molybdate- or tungstate-laden
carrier material and a molybdate- or tungstate-depleted aqueous
solution; c) separating the molybdate- or tungstate-laden carrier
material from the molybdate- or tungstate-depleted aqueous
solution; d) contacting the molybdate- or tungstate-laden carrier
material with an aqueous solution at a pH in the range between 7
and 14, to give a molybdate- or tungstate-depleted carrier material
and a molybdate- or tungstate-laden aqueous solution; e) separating
the molybdate- or tungstate-depleted carrier material from the
molybdate- or tungstate-laden aqueous solution; and f) returning
the molybdate- or tungstate-laden aqueous solution obtained in step
d) to the first bleaching stage, the third bleaching stage or the
first and third bleaching stages.
11. The process of claim 10, wherein the molybdate- or
tungstate-depleted carrier material separated in step e) is used
again in step b).
12. The process of claim 10, wherein in the first and third
bleaching stages the pulp is reacted with 0.1 to 5 wt % of hydrogen
peroxide in the presence of 10 to 2000 ppm of molybdenum in the
form of molybdate or 200 to 10 000 ppm of tungsten in the form of
tungstate, based in each case on the mass of dry pulp, at a
temperature of 50 to 150.degree. C. and at a pH in the range from 1
to 7, and in the second bleaching stage the pulp is reacted with
0.1 to 5 wt % of hydrogen peroxide, based on the mass of dry pulp,
at a temperature of 50 to 100.degree. C. and at a pH in the range
between 7 and 12.
13. The process of claim 12, wherein a complexing agent is added in
the first bleaching stage, in the third bleaching stage or in the
first and third bleaching stages.
14. The process of claim 10, wherein said process does not comprise
further bleaching stages between the first and second bleaching
stages and between the second and third bleaching stages.
15. The process of claim 10, wherein the second bleaching stage is
carried out with addition of oxygen at a pressure of 0.1 to 1.5
MPa.
16. The process of claim 10, wherein a complexing agent is added in
the first bleaching stage, in the third bleaching stage or in the
first and third bleaching stages.
17. The process of claim 10, comprising an additional stage of
alkaline delignification of the pulp with oxygen before the first
bleaching stage.
18. The process of claim 10, comprising an additional stage of
acidic hydrolysis with addition of a complexing agent before the
first bleaching stage.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is US national stage of international
application PCT/EP2014/072096, which had an international filing
date of Oct. 15, 2014, and which was published in German on May 14,
2015. Priority is claimed to German application DE 10 2013 222
468.0, filed on Nov. 6, 2013. The contents of the priority
application is hereby incorporated by reference in its
entirety.
The invention relates to a process for the delignification and
bleaching of pulp with no need for any oxidizing agents other than
oxygen and hydrogen peroxide.
For the manufacture of paper, following pulp cooking, the pulp has
to be delignified and bleached in a plurality of stages. Whereas,
in the past, elemental chlorine was mainly used for the
delignifying and bleaching, it is nowadays preferred in ECF
(Elemental Chlorine-Free) bleaching to employ bleaching sequences
which use chlorine dioxide instead of elemental chlorine. The
bleaching sequence most frequently employed in this case is
ODE.sub.OPDP, where O stands for a delignification with oxygen
under alkaline conditions, D denotes stages with chlorine dioxide
as delignifying and bleaching agent, E.sub.OP stands for an
alkaline extraction with addition of oxygen and hydrogen peroxide,
P denotes a bleaching stage with hydrogen peroxide in the alkaline
range, with the pulp being washed between each of the individual
stages. The nomenclature rules of the "Glossary of Bleaching Terms"
by the Bleaching Committee, Technical Section, Canadian Pulp and
Paper Association (ISBN 1-895288-90-8) are used here and
hereinafter for the coding of bleaching stages and bleaching
sequences by means of letters.
A disadvantage of chlorine dioxide is that it cannot be transported
or stored for a long time, and so, for the bleaching of pulp, it
must be manufactured in a separate unit at the pulp mill.
Consequently, in addition to the costs for the sodium chlorate
starting material, there are also capital costs and operating costs
incurred for such a unit. Moreover, even in the case of
delignification with chlorine dioxide, chlorinated compounds are
formed, and lead to the undesirable presence of organochlorine
substances in the pulp and in the wastewater.
To avoid these disadvantages of chlorine dioxide, the oxidizing
agents ozone and percarboxylic acids, such as peracetic acid and
monoperoxysulphuric acid, have been used as alternative
delignifying agents. These oxidizing agents allow bleaching
sequences for totally chlorine-free (TCF) bleaching, but the pulp
bleached in this way, when bleached to the brightness customary
with ECF bleaching, exhibits poorer mechanical properties, evident
from the significantly lower viscosity of the bleached pulp. The
costs of these delignifying agents are also higher than for
chlorine dioxide.
Another proposed alternative to chlorine dioxide has been a
delignification with hydrogen peroxide in the acidic range in the
presence of molybdate or tungstate as catalyst. U.S. Pat. No.
4,427,490 describes a delignification with hydrogen peroxide under
acidic conditions in the presence of tungstate as catalyst. In
Journal of Pulp and Paper Science Vol. 18 (1992), pages J108-J114,
Kubelka describes a delignification with hydrogen peroxide, carried
out at a pH of 5 using sodium molybdate as catalyst. U.S. Pat. No.
6,165,318 discloses heteropolytungstates and heteropolymolybdates
as catalysts for delignification with hydrogen peroxide in the
acidic range.
It has now been found that with bleaching sequences comprising two
bleaching stages with hydrogen peroxide in the acidic range in the
presence of a molybdate or tungstate and an intervening bleaching
stage with hydrogen peroxide in the alkaline range, it is possible
to produce a bleached pulp which by comparison with an ECF-bleached
pulp exhibits no disadvantages in brightness and viscosity and has
less of a tendency towards yellowing.
The invention accordingly provides a process for the
delignification and bleaching of pulp, comprising a first bleaching
stage with hydrogen peroxide in the presence of a molybdate or
tungstate in an acidic aqueous mixture; subsequent to the first
bleaching stage, a second bleaching stage with hydrogen peroxide in
an alkaline aqueous mixture; and, subsequent to the second
bleaching stage, a third bleaching stage with hydrogen peroxide in
the presence of a molybdate or tungstate in an acidic aqueous
mixture.
In the first bleaching stage of the process of the invention, the
pulp is reacted with hydrogen peroxide in the presence of a
molybdate or tungstate. Hydrogen peroxide is used preferably in an
amount of 0.1 to 5 wt %, based on the mass of dry pulp employed.
More preferably 0.2 to 2 wt % and most preferably 0.5 to 1 wt % of
hydrogen peroxide are used. Hydrogen peroxide is used preferably in
the form of an aqueous solution having a hydrogen peroxide content
of 35 to 70 wt %.
In the first bleaching stage the reaction with hydrogen peroxide
takes place in the presence of a molybdate or tungstate, which acts
as catalyst for the hydrogen peroxide bleaching. The terms
molybdate and tungstate in accordance with the invention encompass
not only mononuclear molybdates and tungstates, such as
MoO.sub.4.sup.2- or WO.sub.4.sup.2-, but also polynuclear
molybdates and tungstates, such as Mo.sub.7O.sub.24.sup.6-,
Mo.sub.8O.sub.26.sup.4-, HW.sub.6O.sub.21.sup.5-,
W.sub.12O.sub.41.sup.10- or W.sub.12O.sub.39.sup.6-, and
polynuclear molybdates and tungstates containing heteroatoms, such
as PMo.sub.12O.sub.40.sup.3-, SiMO.sub.12O.sub.40.sup.3-,
PW.sub.12O.sub.40.sup.3- or SiW.sub.12O.sub.40.sup.3-. When using
molybdate as catalyst, the molybdate is employed preferably in an
amount of 10 to 2000 ppm, more preferably 100 to 1500 ppm and most
preferably 200 to 600 ppm of molybdenum, based on the mass of the
dry pulp. When using tungstate as catalyst, the tungstate is used
preferably in an amount of 200 to 10 000 ppm, preferably 500 to
1500 ppm and most preferably 1500 to 3000 ppm of tungsten, based on
the mass of dry pulp. In accordance with the nomenclature rules
referred to above, the first bleaching stage is designated Pmo if
molybdate is used as catalyst, and Pw if tungstate is used as
catalyst.
The molybdate or tungstate used as catalyst may be added before or
after the hydrogen peroxide or at the same time as the hydrogen
peroxide. In a preferred embodiment, the molybdate or tungstate and
the hydrogen peroxide are added at the same time but separately
from one another in the form of two aqueous solutions.
By choosing the amounts of hydrogen peroxide and molybdate in the
preferred ranges, a particularly effective delignification and
bleaching of the pulp is achieved, and a pulp is obtained that has
a reduced yellowing tendency.
In the first bleaching stage of the process of the invention, the
reaction of the pulp with hydrogen peroxide takes place preferably
at a temperature of 50 to 150.degree. C., more preferably of 60 to
120.degree. C. and most preferably of 70 to 90.degree. C. The
reaction of the pulp with hydrogen peroxide takes place preferably
for a time of 60 to 180 minutes, more preferably 90 to 120
minutes.
The reaction of the pulp in the first bleaching stage takes place
in an acidic aqueous mixture. The reaction takes place preferably
at a pH of the aqueous mixture in the range from 1 to 7, more
preferably 2 to 5 and most preferably 2 to 4. This range for the pH
refers to pH values measured at the end of the bleaching stage at
the temperature of the reaction. The pH of the aqueous mixture is
adjusted preferably by addition of an inorganic acid, more
preferably by addition of sulphuric acid or hydrochloric acid.
In the first bleaching stage the reaction of the pulp takes place
preferably at a pulp density in the range from 3% to 30%, i.e. in
an aqueous mixture having a pulp content of 3 to 30 wt %,
calculated as dry pulp relative to the total mass of the aqueous
mixture. The pulp density is more preferably in the range from 5%
to 20% and most preferably in the range from 8% to 15%.
In the second bleaching stage of the process of the invention, the
pulp is reacted with hydrogen peroxide in an alkaline aqueous
mixture. The reaction takes place preferably at a pH of the aqueous
mixture in the range between 7 and 12, more preferably 8 to 11 and
most preferably 9 to 11. This range for the pH refers to pH values
measured at the end of the bleaching stage at the temperature of
the reaction. The pH of the aqueous mixture is adjusted preferably
by addition of an inorganic base, more preferably by addition of
sodium hydroxide. Hydrogen peroxide is used preferably in an amount
of 0.1 to 5 wt %, based on the mass of dry pulp employed. With
particular preference 0.2 to 2 wt % and most preferably 0.5 to 1 wt
% of hydrogen peroxide are used. The reaction of the pulp with
hydrogen peroxide takes place preferably at a temperature of 50 to
100.degree. C., more preferably of 60 to 100.degree. C. and most
preferably of 70 to 90.degree. C. In accordance with the
nomenclature rules referred to above, the second bleaching stage is
designated Ep when its primary result is an extraction of
alkali-soluble lignin degradation products formed in the first
bleaching stage, and by P when its primary result is a bleaching of
the pulp.
The second bleaching stage may take place with addition of oxygen.
Oxygen in this case is used preferably in the form of substantially
pure oxygen or in the form of oxygen-enriched air. When oxygen is
added, the second bleaching stage is carried out preferably at a
pressure of 0.1 to 1.5 MPa, more preferably at 0.3 to 1.0 MPa and
most preferably of 0.3 to 0.5 MPa. When oxygen is added, the second
bleaching stage, in accordance with the nomenclature rules referred
to above, is designated Eop when its primary result is an
extraction of alkali-soluble lignin degradation products formed in
the first bleaching stage, by Po when its primary result is a
bleaching of the pulp, and by Op when its primary result is a
delignification of the pulp.
The second bleaching stage may be carried out with addition of a
bleaching catalyst, preferably with addition of one of the
manganese complexes known from WO 97/44520. With particular
preference the binuclear manganese complex with the formula
(Me.sub.2TACN).sub.2Mn.sup.IIIMn.sup.IV(.mu.-0).sub.2(.mu.-OAc)].sup.2+2X-
.sup.-, where Me.sub.2TACN stands for
1,2-bis(4,7-dimethyl-1,4,7-triazacyclononan-1-yl)ethane, OAc stands
for acetate and X.sup.- stands for a monovalent anion, known from
WO 97/44520, is used as a bleaching catalyst. X.sup.- is then
preferably acetate, chloride or hexafluorophosphate.
In the third bleaching stage of the process of the invention, the
pulp is again reacted with hydrogen peroxide in the presence of a
molybdate or tungstate in an acidic aqueous mixture. The preferred
conditions for the reaction in the third bleaching stage correspond
to the preferred conditions for the first bleaching stage. The
third bleaching stage may be carried out under the same conditions
as the first bleaching stage, or under differing conditions, for
example with a smaller amount of hydrogen peroxide.
In a preferred embodiment of the process of the invention, a
complexing agent is added in the first bleaching stage, in the
third bleaching stage or in the first and third bleaching stages.
For this purpose all of the complexing agents known from the prior
art for reducing the decomposition of hydrogen peroxide in pulp
bleaching can be used. Complexing agents used preferably are
aminocarboxylic acids or aminophosphonic acids, more particularly
ethylenediaminetetraacetic acid (EDTA),
diethylenetriaminepentaacetic acid (DTPA),
N-hydroxyethyl-N,N',N'-triacetic acid,
cyclohexanediaminetetraacetic acid, aminotrimethylenephosphonic
acid, ethylenediaminetetramethylenephosphonic acid,
diethylenetriaminepentamethylenephosphonic acid,
propylenediaminetetramethylenephosphonic acid,
dipropylenetriaminepentamethylenephosphonic acid and
1-hydroxyethane-1,1-diphosphonic acid, and also their alkali metal
salts. Other suitable complexing agents are ion exchangers based on
bentonite, polyoxycarboxylate-polyacrylic acid copolymers, sodium
iminosuccinate, aspartyl diethoxysuccinate, iminodisuccinate,
ethylenediaminedisuccinate, methylglycinediacetic acid,
nitrilotriacetic acid, modified anionic polyamine, and
polyhydroxyacrylic acid. Particularly preferred complexing agents
are EDTA and DTPA and their sodium salts. Complexing agents are
used preferably in an amount of 0.05 to 1 wt %, based on the mass
of dry pulp employed. The addition of a complexing agent allows
better delignification and bleaching to be achieved for a given
amount of hydrogen peroxide, or allows a reduction in the amount of
hydrogen peroxide needed for achieving a desired degree of
delignification and bleaching.
In the three bleaching stages of the process of the invention,
further to the substances already identified, it is possible to
employ further stabilizers, known from the prior art, for hydrogen
peroxide bleaching, examples being waterglass and magnesium
sulphate.
The pulp is preferably washed after the first bleaching stage and
after the second bleaching stage. For this purpose the mixture
resulting from the bleaching stage is preferably dewatered by
filtration with a drum filter, a filter press or a screw press and
is subsequently admixed with water to set the pulp density desired
for the next bleaching stage. Alternatively or additionally it is
possible to carry out a displacement wash with water on the filter.
Washing the pulp keeps down the consumption of bleaching agent and
auxiliaries for adjusting the pH in the second and third bleaching
stages.
The process of the invention preferably comprises no further
bleaching stages between the first and second bleaching stages and
between the second and third bleaching stages. Corresponding
preferred embodiments comprise the bleaching sequences PmoPPmo,
PmoEpPmo, PmoPoPmo, PmoEopPmo, PmoOpPmo, PwPPw, PwEpPw, PwPoPw,
PwEopPw and PwOpPw.
The process of the invention preferably comprises no further
bleaching stages in which an oxidizing agent other than hydrogen
peroxide and oxygen is used. The limitation to hydrogen peroxide
and oxygen as oxidizing agents has the advantage that no toxic
bleaching agents are required for the process, and that only
storable bleaching agents are used.
In a preferred embodiment the process of the invention comprises,
before the first bleaching stage, an additional stage of alkaline
delignification of the pulp with oxygen, which is carried out
preferably under pressure. The alkaline delignification with oxygen
is preferably the first delignification stage in the process.
Corresponding preferred embodiments comprise the bleaching
sequences OPmoPPmo, OPmoEpPmo, OPmoPoPmo, OPmoEopPmo, OPmoOpPmo,
OPwPPw, OPwEpPw, OPwPoPw, OPwEopPw and OPwOpPw. An upstream
alkaline delignification with oxygen allows the oxygen consumption
for the process of the invention to be reduced. Suitable conditions
for alkaline delignification with oxygen are known to the skilled
person from the prior art.
In another preferred embodiment, the process of the invention,
after the third bleaching stage, comprises an additional bleaching
stage with hydrogen peroxide in an alkaline aqueous mixture. The
preferred conditions for the reaction in this additional bleaching
stage correspond to the preferred conditions for the second
bleaching stage.
The additional bleaching stage may be carried out under the same
conditions as the second bleaching stage or under differing
conditions, as for example with additional addition of oxygen.
Corresponding preferred embodiments comprise the bleaching
sequences PmoPPmoP, PmoEpPmoP, PmoPoPmoP, PmoEopPmoP, PmoOpPmoP,
PwPPwP, PwEpPwP, PwPoPwP, PwEopPwP, PwOpPwP, PmoPPmoPo, PmoEpPmoPo,
PmoPoPmoPo, PmoEopPmoPo, PmoOpPmoPo, PwPPwPo, PwEpPwPo, PwPoPwPo,
PwEopPwPo and PwOpPwPo. The additional bleaching stage with
hydrogen peroxide after the third bleaching stage is preferably
combined with an alkaline delignification with oxygen that is
carried out before the first bleaching stage. Corresponding
preferred embodiments comprise the bleaching sequences OPmoPPmoP,
OPmoEpPmoP, OPmoPoPmoP, OPmoEopPmoP, OPmoOpPmoP, OPwPPwP, OPwEpPwP,
OPwPoPwP, OPwEopPwP, OPwOpPwP, OPmoPPmoPo, OPmoEpPmoPo,
OPmoPoPmoPo, OPmoEopPmoPo, OPmoOpPmoPo, OPwPPwPo, OPwEpPwPo,
OPwPoPwPo, OPwEopPwPo and OPwOpPwPo. Bleaching sequences of this
embodiment are suitable in particular for the delignification and
bleaching of softwood pulp.
In a likewise preferred embodiment, the process of the invention,
before the first bleaching stage, comprises an additional stage of
acidic hydrolysis with addition of at least one complexing agent.
Complexing agents which can be used for this purpose are the
compounds listed earlier on above for addition in the first or
third bleaching stage. Complexing agents in this case are used
preferably in an amount of 0.01 to 1 wt %, more preferably 0.1 to
0.5 wt %, based on the mass of dried pulp employed. The acidic
hydrolysis is carried out preferably at a pH of the aqueous mixture
in the range from 2 to 7, more preferably 3 to 6. This range for
the pH refers to pH values measured at the end of the hydrolysis
stage at the temperature of the reaction. The pH is adjusted
preferably by addition of an inorganic acid, more preferably by
addition of sulphuric acid or hydrochloric acid. The acidic
hydrolysis is carried out preferably at a temperature of 50 to
100.degree. C., more preferably at 60 to 90.degree. C., preferably
for a time of 60 to 480 minutes, more preferably 120 to 320
minutes, and preferably at a pulp density in the range from 2% to
30%, more preferably 5% to 15%. In accordance with the nomenclature
rules referred to above, the acidic hydrolysis with addition of a
complexing agent is designated Aq. Corresponding preferred
embodiments comprise the bleaching sequences AqPmoPPmo, AqPmoEpPmo,
AqPmoPoPmo, AqPmoEopPmo, AqPmoOpPmo, AqPwPPw, AqPwEpPw, AqPwPoPw,
AqPwEopPw and AqPwOpPw. This embodiment may also be combined with
the embodiment of a prior alkaline delignification of the pulp with
oxygen, to give the bleaching sequences OAqPmoPPmo, OAqPmoEpPmo,
OAqPmoPoPmo, OAqPmoEopPmo, OAqPmoOpPmo, OAqPwPPw, OAqPwEpPw,
OAqPwPoPw, OAqPwEopPw and OAqPwOpPw. This embodiment may also be
combined with the embodiment of a downstream additional alkaline
bleaching stage with hydrogen peroxide, to give the bleaching
sequences AqPmoPPmoP, AqPmoEpPmoP, AqPmoPoPmoP, AqPmoEopPmoP,
AqPmoOpPmoP, AqPwPPwP, AqPwEpPwP, AqPwPoPwP, AqPwEopPwP, AqPwOpPwP,
AqPmoPPmoPo, AqPmoEpPmoPo, AqPmoPoPmoPo, AqPmoEopPmoPo,
AqPmoOpPmoPo, AqPwPPwPo, AqPwEpPwPo, AqPwPoPwPo, AqPwEopPwPo,
AqPwOpPwPo, OAqPmoPPmoP, OAqPmoEpPmoP, OAqPmoPoPmoP, OAqPmoEopPmoP,
OAqPmoOpPmoP, OAqPwPPwP, OAqPwEpPwP, OAqPwPoPwP, OAqPwEopPwP,
OAqPwOpPwP, OAqPmoPPmoPo, OAqPmoEpPmoPo, OAqPmoPoPmoPo,
OAqPmoEopPmoPo, OAqPmoOpPmoPo, OAqPwPPwPo, OAqPwEpPwPo,
OAqPwPoPwPo, OAqPwEopPwPo and OAqPwOpPwPo. Furthermore, The stage
of acidic hydrolysis with addition of a complexing agent may also
be combined with a subsequent alkaline bleaching stage with
hydrogen peroxide, to give the bleaching sequences AqPPmoPPmo,
AqPPmoEpPmo, AqPPmoPoPmo, AqPPmoEopPmo, AqPPmoOpPmo, AqPPwPPw,
AqPPwEpPw, AqPPwPoPw, AqPPwEopPw, AqPPwOpPw, OAqPPmoPPmo,
OAqPPmoEpPmo, OAqPPmoPoPmo, OAqPPmoEopPmo, OAqPPmoOpPmo, OAqPPwPPw,
OAqPPwEpPw, OAqPPwPoPw, OAqPPwEopPw, OAqPPwOpPw, AqPPmoPPmoP,
AqPPmoEpPmoP, AqPPmoPoPmoP, AqPPmoEopPmoP, AqPPmoOpPmoP, AqPPwPPwP,
AqPPwEpPwP, AqPPwPoPwP, AqPPwEopPwP, AqPPwOpPwP, AqPPmoPPmoPo,
AqPPmoEpPmoPo, AqPPmoPoPmoPo, AqPPmoEopPmoPo, AqPPmoOpPmoPo,
AqPPwPPwPo, AqPPwEpPwPo, AqPPwPoPwPo, AqPPwEopPwPo, AqPPwOpPwPo,
OAqPPmoPPmoP, OAqPPmoEpPmoP, OAqPPmoPoPmoP, OAqPPmoEopPmoP,
OAqPPmoOpPmoP, OAqPPwPPwP, OAqPPwEpPwP, OAqPPwPoPwP, OAqPPwEopPwP,
OAqPPwOpPwP, OAqPPmoPPmoPo, OAqPPmoEpPmoPo, OAqPPmoPoPmoPo,
OAqPPmoEopPmoPo, OAqPPmoOpPmoPo, OAqPPwPPwPo, OAqPPwEpPwPo,
OAqPPwPoPwPo, OAqPPwEopPwPo and OAqPPwOpPwPo. The use of an
additional stage of acidic hydrolysis with addition of at least one
complexing agent before the first bleaching stage has particular
advantages in the bleaching of hardwood pulp, and reduces the
consumption of oxidizing agent in the subsequent bleaching
stages.
The molybdate or tungstate used as catalyst in the first and third
bleaching stages of the process of the invention is preferably
recovered and returned to the bleaching stages. Methods
particularly suitable for this purpose are those known from WO
2009/133053 and WO 2013/110419. In a preferred embodiment,
therefore, the process of the invention comprises the additional
steps of a) separating the pulp from the aqueous mixture subsequent
to the first bleaching stage, the third bleaching stage or the
first and third bleaching stages to give a molybdate- or
tungstate-containing aqueous solution, b) contacting the molybdate-
or tungstate-containing aqueous solution obtained in step a) with a
water-insoluble, cationized inorganic carrier material at a pH in
the range between 2 and 7, to give a molybdate- or tungstate-laden
carrier material and a molybdate- or tungstate-depleted aqueous
solution, c) separating the molybdate- or tungstate-laden carrier
material from the molybdate- or tungstate-depleted aqueous
solution, d) contacting the molybdate- or tungstate-laden carrier
material with an aqueous solution at a pH in the range between 7
and 14, to give a molybdate- or tungstate-depleted carrier material
and a molybdate- or tungstate-laden aqueous solution, e) separating
the molybdate- or tungstate-depleted carrier material from the
molybdate- or tungstate-laden aqueous solution, and f) returning
the molybdate- or tungstate-laden aqueous solution obtained in step
d) to the first bleaching stage, the third bleaching stage or the
first and third bleaching stages.
In step a) the delignified pulp is separated from the mixture
obtained in the first bleaching stage, in the third bleaching stage
or in the first and third bleaching stages, to give a molybdate- or
tungstate-containing aqueous solution. The separation is
accomplished preferably by filtration, more particularly by
filtration with a drum filter, a filter press or a screw press.
Suitable filtration techniques are known to the person skilled in
the art of pulp bleaching.
In step b), the molybdate- or tungstate-containing aqueous solution
obtained in step a) is contacted at a pH in the range between 2 and
7 with a water-insoluble, cationized inorganic carrier material.
The pH is adjusted preferably to a level in the range from 3 to 5,
more preferably in the range from 3.5 to 4. Adjustment to a pH
within these ranges permits almost complete recovery of molybdate
or tungstate from the aqueous solution with little consumption of
pH-regulating agents. For the contacting, the water-insoluble,
cationized inorganic carrier material is preferably dispersed with
a stirrer or a disperser in the molybdate- or tungstate-containing
aqueous solution. The contacting may take place at any desired
temperature, suitable temperatures being in the range from 0 to
100.degree. C. In step b) the cationized inorganic carrier material
is used for contacting with the molybdate- or tungstate-containing
aqueous solution, preferably in an amount of 10 to 1000 parts by
weight of carrier material per part by weight of molybdenum or in
an amount of 200 to 10 000 parts by weight of carrier material per
part of weight of tungsten. For the recovery of molybdate, more
preferably 50 to 500 and more particularly 100 to 300 parts by
weight of carrier material are used per part of weight of
molybdenum. For the recovery of tungstate, more preferably 1000 to
5000 and more particularly 2000 to 3000 parts by weight of carrier
material are used per part by weight of tungsten.
Inorganic carrier materials, whose surface has been modified with
positively charged functional groups, are suitable as cationized
inorganic carrier material. The modification may take place, for
example, by reaction of the surface with reagents which anchor a
positively charged functional group covalently on the surface.
Suitable water-insoluble, cationized inorganic carrier materials
with covalently anchored, positively charged functional groups are,
for example, precipitated or fumed silicas which have been modified
with aminosilanes and preferably also quaternized on the amino
group. The modification may also take place, alternatively, by ion
exchange of an inorganic carrier material, negatively charged on
the surface, with a quaternary ammonium salt. The quaternary
ammonium salt used for this purpose preferably has at least one
non-polar alkyl radical having 6 to 24, more preferably 12 to 22,
carbon atoms, in order to prevent detachment of the quaternary
ammonium ions from the carrier in the acidic range.
A cationized phyllosilicate is preferably used as water-insoluble,
cationized inorganic carrier material, more preferably a
phyllosilicate ion-exchanged with a quaternary ammonium salt.
Suitable phyllosilicates here include kaolins, smectites, illites,
bentonites (montmorillonites), hectorites, pyrophyllites,
attapulgites, sepiolites and laponites, preferably bentonites,
hectorites and attapulgites ion-exchanged with a quaternized
ammonium salt, more preferably bentonite ion-exchanged with
quaternary ammonium salt.
Bentonites, hectorites and attapulgites ion-exchanged with
quaternized ammonium salts are available commercially:
Quaternium-18 Bentonite as Bentone 34 from Rheox Corp. and as
Claytone 34, Claytone 40 and Claytone XL from Southern Clay;
Stearalkonium Bentonite as Tixogel LG from United Catalysts, as
Bentone SD-2 from Elementis Specialties and as Claytone AF and
Claytone APA from Southern Clay; Quaternium-18/Benzalkonium
Bentonite as Claytone GR, Claytone HT and Claytone PS from Southern
Clay; Quaternium-18 Hectorite as Bentone 38 from Rheox Corp.;
Dihydrogenated Tallow Benzylmonium Hectorite as Bentone SD-3 from
Rheox Corp.; Stearalkonium Hectorite as Bentone 27 from Rheox
Corp.; and Cationized Attapulgite as Vistrol 1265 from Cimbar.
These ion-exchanged phyllosilicates may be used both as powder and
in the form of the commercially available dispersions in an oil or
an organic solvent.
Besides the commercial bentonites, hectorites and attapulgites
ion-exchanged with tetraalkylammonium ions, it is also possible to
employ the corresponding materials ion-exchanged with quaternized
alkanolamine fatty acid esters, more particularly bentonite
ion-exchanged with dimethyldiethanolammonium mono- and difatty acid
esters, and also methyltriethanolammonium mono-, di- and tri-fatty
acid esters. Preference here is given to using corresponding esters
with saturated fatty acids, especially saturated fatty acids having
12 to 18 carbon atoms.
In step c) the molybdate- or tungstate-laden carrier material is
separated from the molybdate- or tungsten-depleted aqueous
solution. The separation may take place with any of the
solids/liquids separation methods known to the skilled person, as
for example by sedimentation, filtration, centrifugation or
flotation. The separated molybdate- or tungstate-depleted carrier
material may additionally be washed with an aqueous solution having
a pH of between 6 and 14, in order to complete the detachment of
molybdate or tungstate from the carrier material. The wash liquor
resulting from the washing is preferably combined with the
molybdate- or tungstate-laden solution.
In step d), the molybdate- or tungstate-laden carrier material is
contacted with an aqueous solution having a pH in the range between
7 and 14. This pH is selected preferably in the range from 8 to 12
and more preferably in the range from 9 to 11. The contacting may
take place at any desired temperature, suitable temperatures being
in the range from 0 to 100.degree. C.
In step e) the molybdate- or tungstate-depleted carrier material is
separated from the molybdate- or tungstate-laden aqueous solution.
The separation may take place with any of the solids/liquids
separation methods known to the skilled person, as for example by
sedimentation, filtration, centrifugation or flotation. The
separated molybdate- or tungstate-depleted carrier material may
additionally be washed with an aqueous solution having a pH of
between 6 and 14, in order to complete the detachment of molybdate
or tungstate from the carrier material. The wash liquor resulting
from the washing is preferably combined with the molybdate- or
tungstate-laden solution. The molybdate- or tungstate-depleted
carrier material removed in step e) is preferably used again in
step b).
In a preferred embodiment the water-insoluble, cationized inorganic
carrier material is arranged in a fixed bed. Steps b) and c) are
then accomplished by passing the molybdate- or tungstate-containing
aqueous solution through a fixed bed comprising the
water-insoluble, cationized inorganic carrier material. As the
molybdate- or tungstate-containing aqueous solution passes through
the fixed bed, the molybdate or tungstate present in the solution
already becomes bound to the water-insoluble, cationized inorganic
carrier material, and the aqueous solution leaving the fixed bed is
molybdate- or tungstate-depleted. After the loading of the
water-insoluble cationized inorganic carrier material arranged in
the fixed bed, steps d) and e) are carried out by passing an
aqueous solution having a pH in the range between 6 and 14 through
the fixed bed loaded with molybdate or tungstate in steps b) and
c). The aqueous solution leaving the fixed bed here comprises the
major fraction of the molybdate or tungstate bound in step b) to
the water-insoluble, cationized inorganic carrier material, and,
after these steps have been carried out, the fixed bed can be used
again for the recovery of molybdate or tungstate in steps b) and
c). The passing of the molybdate- or tungstate-containing aqueous
solution through the fixed bed is preferably ended before the
amount of molybdate or tungstate in the aqueous solution leaving
the fixed bed rises above the desired residual level. The fixed bed
preferably further comprises a water-insoluble packing material for
increasing the porosity of the fixed bed in addition to the
water-insoluble, cationized inorganic carrier material. Suitable
water-insoluble packing materials are known from WO 2009/133053.
The fixed bed comprises the water-insoluble, cationized inorganic
carrier material and the water-insoluble packing material
preferably in a weight ratio of from 10:1 to 1:100. With preference
at least two fixed beds arranged in parallel are used, in which
steps b) and c) and steps d) and e) are carried out in
alternation--in other words, in a first fixed bed, molybdate or
tungstate is recovered from an aqueous solution in steps b) and c),
while in a second fixed bed, arranged in parallel and already laden
with molybdate or tungstate, the molybdate or tungstate is detached
again from the carrier in steps d) and e). In a particularly
preferred embodiment, switching then takes place between the
parallel fixed beds in such a way that the passage of the
molybdate- or tungstate-containing aqueous solution through a fixed
bed takes place continuously.
In step f) the molybdate- or tungstate-laden aqueous solution
obtained in step d) is returned to the first bleaching stage, to
the third bleaching stage or to the first and third bleaching
stages.
Preferably, molybdate or tungstate is removed both from the aqueous
mixture obtained in the first bleaching stage and from the aqueous
mixture obtained in the third bleaching stage in two parallel steps
a). In that case the recovery of molybdate or tungstate may be
carried out in such a way that steps b) to f) are carried out each
separately from one another with the molybdate- or
tungstate-containing aqueous solutions obtained in the two steps
a). With this embodiment, in the respective step f), the molybdate-
or tungstate-laden aqueous solution is preferably returned to the
bleaching stage from which the molybdate or tungstate was removed
in the respective step a). Preferably, however, the molybdate- or
tungstate-containing aqueous solutions obtained in two parallel
steps a) are combined with one another, then steps b) to e) are
carried out, and in step f) the molybdate- or tungstate-laden
aqueous solution obtained in step e) is divided in accordance with
the amount of catalyst desired in the respective bleaching stages,
and is returned to the first and third bleaching stages.
The examples which follow illustrate the invention, but without
restricting the subject matter of the invention.
EXAMPLES
All of the experiments were carried out with kraft pulps which had
been delignified with oxygen under alkaline conditions. Examples 1
to 4 used an oxygen-delignified eucalyptus kraft pulp with a
brightness of 64.7% ISO, Examples 5 and 6 an oxygen-delignified
spruce kraft pulp with a brightness of 48.1% ISO.
The bleaching stages were each carried out under the experimental
conditions indicated, with the pulp densities specified in Tables 1
to 6, the pulp being mixed with the corresponding amount of water
and with the amounts of bleaching chemicals indicated in the
tables, and maintained at the stated temperature in a plastics
pouch within a thermostated waterbath. In a deviation from this
procedure, in Examples 5 and 6, the alkaline, oxygen- and
peroxide-assisted extraction Eop, the peroxide-assisted oxygen
stage Op and the oxygen-assisted peroxide stage Po were carried out
in a high-shear mixer at the oxygen pressure stated in each case.
The amounts of bleaching chemicals stated are based on the mass of
the dry pulp used in the bleaching sequence. In the case of EDTA,
the amounts are based on the amount of the commercial 40 wt %
aqueous solution used. For the catalysed bleaching with hydrogen
peroxide in the presence of molybdate, the catalyst used was sodium
molybdate in the form of an aqueous solution. The pH values at the
start of a bleaching stage were determined at room temperature; the
pH values at the end of the bleaching stage were determined at the
temperature of the bleaching stage, in each case using a glass
combination electrode.
Washing took place between each of the bleaching stages, by adding
demineralised water to a pulp density of 2 wt %, intensive stirring
of the resulting suspension and removal of the pulp from this
suspension by means of vacuum filtration and centrifugation.
TABLE-US-00001 TABLE 1 Bleaching of oxygen-delignified eucalyptus
kraft pulp in Example 1 with the bleaching sequence DEpDP
Quantities used and bleaching Bleaching stage parameters D Ep D P
ClO.sub.2 in wt % of 2.6 0.5 active chlorine H.sub.2SO.sub.4 in wt
% 0.4 0.1 H.sub.2O.sub.2 in wt % 0.4 0.2 NaOH in wt % 0.4 0.4
Temperature in 90 85 80 80 .degree. C. Time in minutes 120 75 120
120 Pulp density in % 10 10 10 10 pH at start 10.7 11.2 pH at end
2.8 9.0 4.3 10.2
TABLE-US-00002 TABLE 2 Bleaching of oxygen-delignified eucalyptus
kraft pulp in Example 2 with the bleaching sequence AqPPmoPPmoP
Quantities employed and bleaching Bleaching stage parameters Aq P
Pmo P Pmo P H.sub.2O.sub.2 in wt % 2.0 0.5 2.0 0.1 2.0
H.sub.2SO.sub.4 in 0.25 0.28 0.28 wt % NaOH in wt % 1.4 1.4 1.4 Mo
in wt % 0.025 0.01 EDTA in wt % 0.2 0.1 0.1 Temperature 90 85 90 85
90 85 in .degree. C. Time in 300 90 120 90 60 90 minutes Pulp 10 10
10 10 10 10 density in % pH at start 4.2 11.6 3.5 11.8 3.5 11.5 pH
at end 4.0 10.7 4.0 10.5 4.1 10.7
TABLE-US-00003 TABLE 3 Bleaching of oxygen-delignified eucalyptus
kraft pulp in Example 3 with the bleaching sequence DEpDP
Quantities employed and bleaching Bleaching stage parameters D Ep D
P ClO.sub.2 in wt % of 1.86 0.2 active chlorine H.sub.2SO.sub.4 in
wt % 0.5 0.15 H.sub.2O.sub.2 in wt % 0.2 0.2 NaOH in wt % 0.4 0.4
Temperature in 90 85 80 80 .degree. C. Time in minutes 120 75 120
120 Pulp density in % 10 10 10 10 pH at start 10.8 11.4 pH at end
2.8 9.1 4.5 10.2
TABLE-US-00004 TABLE 4 Bleaching of oxygen-delignified eucalyptus
kraft pulp in Example 4 with the bleaching sequence AqPPmoPPmoP
Quantities employed and bleaching Bleaching stage parameters Aq P
Pmo P Pmo P H.sub.2O.sub.2 in wt % 0.6 0.5 0.6 0.5 0.6
H.sub.2SO.sub.4 in 0.25 0.35 0.35 wt % NaOH in wt % 1.3 1.3 1.3 Mo
in wt % 0.01 0.01 EDTA in wt % 0.2 0.1 0.1 Temperature 90 85 90 85
90 85 in .degree. C. Time in 300 240 120 240 120 240 minutes Pulp
10 10 10 10 10 10 density in % pH at start 4.6 11.9 4.1 11.9 3.5
11.9 pH at end 4.7 10.7 4.3 10.7 3.9 10.7
TABLE-US-00005 TABLE 5 Bleaching of oxygen-delignified spruce kraft
pulp in Example 5 of the bleaching sequence DEopD Quantities
employed and Bleaching stage bleaching parameters D Eop D ClO.sub.2
in wt % of 2.95 1.0 active chlorine H.sub.2SO.sub.4 in wt % 0.15
0.03 H.sub.2O.sub.2 in wt % 0.5 NaOH in wt % 1.0 O.sub.2 in MPa 0.3
MgSO.sub.4 in wt % 0.1 Temperature in .degree. C. 90 80 75 Time in
minutes 60 90 120 Pulp density in % 10 11 10 pH at start pH at end
2.8 10.8 3.9
TABLE-US-00006 TABLE 6 Bleaching of oxygen-delignified spruce kraft
pulp in Example 6 with the bleaching sequence PmoOpPmoPo Quantities
employed and Bleaching stage bleaching parameters Pmo Op Pmo Po
H.sub.2O.sub.2 in wt % 0.5 0.7 0.5 2.9 H.sub.2SO.sub.4 in wt % 0.25
0.25 NaOH in wt % 1.0 1.6 O.sub.2 in MPa 0.5 0.5 Mo in wt % 0.02
0.02 EDTA in wt % 0.1 0.1 MgSO.sub.4 in wt % 0.1 0.15 Temperature
in 90 100 90 107 .degree. C. Time in minutes 120 75 120 160 Pulp
density in % 10 11 10 12 pH at start 4.7 -- 4.3 -- pH at end 5.2
10.4 5.1 10.8
For the bleached pulp, the brightness of the pulp was determined in
accordance with the PAPTAC Standard E.1, and the viscosity of the
pulp in accordance with TAPPI Standard T 236 om 99. In addition,
the loss of brightness by heat ageing and the post-colour number
(PC number) were determined using the TAPPI T 260 (wet) and TAPPI
UM 200 (dry) methods. The results are summarized in Table 7.
In the pairs of experiments 1 and 2, 3 and 4, and 5 and 6, the
conditions of the bleaching sequences were selected such that the
pulp was bleached in each case to a comparable brightness. For
eucalyptus kraft pulp, the bleaching sequence of the invention, by
comparison with the industrial standard bleaching sequence with
chlorine dioxide, achieves a lower level of fibre damage, evident
from a higher viscosity. Furthermore, the pulps bleached with the
process of the invention exhibited a better stability of
brightness, i.e. a lower yellowing tendency, than the pulps
bleached with chlorine dioxide.
TABLE-US-00007 TABLE 7 Properties of the pulps bleached in Examples
1 to 6 Example 1* 2 3* 4 5* 6 Bleaching DE.sub.pDP
A.sub.qP--P.sub.moPP.sub.moP DE.sub.pDP A.sub.qP--P.s-
ub.moPP.sub.moP DE.sub.opD P.sub.moO.sub.pP.sub.moP.sub.o sequence
Brightness 91.8 91.4 90.1 89.5 87.3 87.2 in % ISO Viscosity 14.4
16.0 15.0 17.2 15.2 12.7 in mPa*s Heat ageing, wet: Change in -2.5
-1.4 -2.7 -0.9 -3.4 -2.3 brightness in % ISO PC number 0.275 0.147
0.364 0.117 0.621 0.403 Heat ageing, dry: Change in -1.9 -2.0 -1.9
-1.4 -2.8 -2.3 brightness in % ISO PC number 0.201 0.224 0.245
0.188 0.498 0.403 number *not according to the invention
* * * * *